This invention relates to two-stroke internal combustion engines, and is concerned with improvements to the porting arrangements of such engines.
In a two-stroke engine, during the working stroke of the piston fresh mixture of air and fuel is being compressed in the crankcase by the downward movement of the piston driven by the expansion of the charge ignited in the combustion chamber. When the piston reaches a predetermined point in its working stroke it uncovers one or more exhaust ports in the cylinder wall through which the expanded combustion gases are discharged to atmosphere, and shortly afterwards it uncovers transfer (scavenge) ports at the ends of passages in the wall of the cylinder which place the crackcase interior in communication with the cylinder space above the piston crown, so that a fresh charge of mixture previously compressed in the crankcase will flow through the transfer ports into the upper working space in the cylinder above the piston. After a suitable period the transfer ports and the exhaust ports are reclosed by the piston wall as the piston moves upwardly on the compression stroke and compresses the fresh charge in the cylinder prior to its ignition and the start of the next working stroke.
Fresh mixture is sucked into the crankcase as the piston rises in its compression stroke, through inlet ports controlled by the skirt of the piston or through non-return valves or timed valves in the crankcase wall, and shortly after the piston reaches its top-dead-centre position these inlet ports or valves close to trap the fresh mixture in the contained volume afforded by the crankcase and the lower part of the cylinder below the piston, in readiness for compression by the descending piston during the next working stroke of the cycle.
In practice it is necessary to design the number, size and disposition of the transfer ports and exhaust ports, and their respective passages leading to the ports, including their inclination, in such a way that as much as possible of the exhaust gases from each cycle are swept out or "scavenged" through the exhaust ports, in order that as much fresh mixture as possible will be retained in the cylinder to produce power in the next cycle. At the same time it is desirable that as little of the fresh mixture as possible should short-circuit from the transfer ports to the exhaust ports before ignition, in the interests of good fuel economy and low exhaust emission.
A secondary, but vital, function of the transfer ports is to create a pattern of internal mixture movement (gas flow) in the combustion chamber and the upper part of the cylinder, which is favourable to efficient and regular combustion.
In the case of a two-stroke engine whose fuel supply is controlled by a carburettor, the power output is controlled by throttling the mixture intake immediately downstream of the carburettor. This produces a sub-atmospheric gas pressure in the crankcase prior to compression by the working stroke, and the crankcase pressures after compression are somewhat lower than those prevailing at full throttle, i.e. full-load operation. The porting details in such engines are usually designed to produce maximum power output when running at full throttle, and a lighter load operation the flow velocities through the transfer ports will be reduced due to the lower crankcase pressures referred to. The Applicant believes that the poor low-load running of this type of two-stroke engine is due to excessive dilution of the fresh charge by the inevitably higher proportion of exhaust residuals left in the cylinder. In particular the amount of fresh charge at a readily-ignitable mixture strength reaching that part of the combustion chamber where ignition is initiated is inadequate for regular combustion, due to the reduced gas flow velocities through the cylinder transfer ports.